Rheumatoid Arthritis (RA) is a complex polygenic disorder. On the basis of twin and family studies, it is likely that anywhere from three to ten different genes may be involved in this disease. Only one of these genetic regions, located in the HLA complex on chromosome 6, has been defined. A major challenge for researchers is to identify the remaining regions which are involved in predisposition to RA. The purpose of this contract is to identify 1000 families in which two or more siblings are affected with rheumatoid arthritis. Detailed clinical information on affected siblings will be obtained and entered into a database. In addition, hand x-rays, serum and peripheral blood DNA will be obtained from each affected sibling. DNA will also be obtained from the parents of these siblings, when available. This database, serum and DNA repository will form a resource for the entire scientific community to allow for a comprehensive analysis of genetic susceptibility to rheumatoid arthritis. Access to the database and DNA repository by qualified scientists will be governed by an oversight committee. The following resources will be available at the conclusion of the study: a) Family trees from at least 1000 families with two or more sibs with RA, without personal identifiers; b) DNA samples from affected sib pairs and their parents (when available); c) genotyping data on all families. The genotyping data will be generated by NARAC investigators under a separate RO-1 grant.